In recent years, shoes are generally provided with shock absorbers for mitigating the shock caused by the collision between the ground and shoe soles. In particular, shock absorbers provided in running shoes are required to have a sufficient shock absorption for mitigating a strong shock generated during running. Meanwhile, if the shock absorption of the shock absorbers is excessively high, the energy loss during running increases, and further the stability during running may possibly be impaired in some cases. Therefore, shock absorbers provided in running shoes are required to have a shock absorption that is suitable for running.
It is known that the running form of a runner differs between in the early stage and in the last stage of long-distance running. Specifically, in the last stage of long-distance running in which the runner is tired, the stride length of the runner decreases as compared with that in the early stage of running. Therefore, the vertical loading rate applied to the feet of the runner increases, resulting in an increase in damage to the body of the runner.
In consideration of this fact, shoes for long-distance running are preferably provided with shock absorbers capable of sufficiently mitigating the damage to the runner in the last stage of long-distance running. However, if the shock absorption of shock absorbers is simply increased according to the load in the last stage of long-distance running, the shock absorption is rendered excessively high in the early stage of running, thereby causing a problem of the energy loss or the like. Therefore, for shoes for long-distance running, shock absorbers having a shock absorption that is higher in the last stage of long-distance running than in the early stage of running are required.
However, shock absorbers provided in shoes generally have a constant shock absorption in any stage of long-distance running, and there are few shock absorbers having a shock absorption that changes depending on the state of use.
As a shock absorber having a shock absorption that changes depending on the state of use, a shock absorber using a non-Newtonian fluid, which is disclosed in Patent Literature 1, is known, for example. The shock absorber has a shock absorption that changes corresponding to the shock transmitted from the feet during running. Specifically, the shock absorber exhibits characteristics of softening during walking and hardening during running.
However, the shock absorber of Patent Literature 1 does not consider the necessity of changes in shock absorption corresponding to running distance, and thus changes in shock absorption between in the early stage and in the last stage of long-distance running are insufficient. Therefore, both of a shock absorption required in the early stage of running and a shock absorption required in the last stage of long-distance running cannot be satisfied.